1. Field of the Invention
The present invention relates generally to a cellular communication system used to off-load telephone calls from a main cellular telephone system to an off-load cellular telephone system, and, more particularly, to a method of ensuring incoming call delivery to a mobile telephone located in the off-load cellular system.
2. Description of the Related Art
Cellular mobile telephones (mobile telephones) are typically employed in automobiles, ships or the like, and thus are transportable with the user into various different geographic zones. In each of the different geographic zones, one or more mobile telephone switching offices (MTSO) are provided to complete call connections. Once connected to the MTSO, the mobile telephone may then be connected to another mobile telephone within a specific zone or, through land-based networks such as a public switching telephone network (PSTN), to a land-based telephone or to another cellular mobile telephone in a different zone.
Cellular mobile telephone systems (cellular systems) service specific geographic areas, or zones, each of which is typically divided into a plurality of cells. Each cell includes a stationary transmitter/receiver (transceiver) station coupled to transmit and receive antennas. The transceiver station broadcasts signals within its signal coverage area and provides radio communication connections with mobile telephones physically located within the associated cell. The stationary transceiver stations, in turn, are connected to a MTSO associated with the zone where the transceiver stations are located. The MTSO is generally a computer-based system and provides call processing and switching functions, between and among cells. Typical cellular systems use cells known as macrocells, which provide signal coverage over fairly large geographic areas, often from a radius of one mile to several miles.
Each cell within a specific zone of a cellular system has a number of frequencies assigned to it to establish radio communications with a mobile telephone. The frequencies are divided into control channels, paging channels and voice channels. The paging and control channels are used for the mutual identification between the mobile telephone and the cellular system providing the cellular mobile telephone service. The location or frequency of the control channels contained in the frequency range assigned to each cell identifies the type of cellular system which is being used. Typically, one set or range of frequencies is called an "A " cellular system and a second set of frequencies is called a "B " cellular system. Generally, a specific geographic area contains one of each type of cellular system (i.e., an A and a B cellular system), thereby providing alternative cellular service to that area.
A mobile telephone must identify itself to the cellular system provider before service for the mobile telephone can be established. The communications between a mobile telephone and its respective transceiver station must conform with the industry standard for such communications, set forth in the Mobile Station--Land Station Compatibility Specification EIA/TIA 553 (EIA/TIA 553). The identification process begins using the control channels. When a mobile telephone is first switched on, the mobile telephone scans through the forward control channels and measures the signal strength of each forward control channel. It will then tune to the strongest forward control channel and lock on to the forward control channel message stream. The forward control channel message stream is a continuous flow of data from the transceiver station to the mobile telephone and contains, for example, set-up parameters, global commands and registration identification information. The origination point of the forward control channel message stream is generally the MTSO. When the mobile telephone makes an access attempt, the mobile telephone must access a reverse control channel. A reverse control channel access message is generated by the mobile telephone and broadcasted over the reverse control channel. The reverse control channel access message contains various data which permits the cellular system to identify the mobile telephone and thereby determine whether the mobile telephone should be provided with cellular service.
In accordance with EIA/TIA 553, the mobile telephone will continuously scan for and lock onto the strongest available control channel. In addition, EIA/TIA 553 requires that a mobile telephone rescan the control channels at the end of a power-up process, tuning to the strongest control channel available. EIA/TIA 553 also requires rescanning for the strongest control channel after a mobile telephone accesses a reverse control channel and after an Autonomous Registration by a mobile telephone with its respective transceiver station. Autonomous Registration is a process used to track the location of a mobile telephone within a cellular system in which a mobile telephone periodically identifies itself to an associated transceiver station as being active in the system at the time the Autonomous Registration message is sent. EIA/TIA 553 does not require that a cellular system provide for Autonomous Registration of mobile telephones, but does specify the procedure for Autonomous Registration if it is used.
In a cellular system which implements Autonomous Registration in compliance with EIA/TIA 553, the Autonomous Registration generally works as follows. Included in the forward control channel message stream broadcasted by the transceiver station of each cell in a cellular system is a registration field. Periodically included in the message stream is a REGID value. Optionally included in the message stream as a Global Action Overhead Message is a REGINCR value. The REGID value is received by the mobile telephone and stored in a designated location where it can then be monitored by the mobile telephone. For example, when a mobile telephone is first powered on, the mobile telephone locks onto the forward control channel message stream of the selected control channel and then receives and stores a REGID value.
As the REGID value is periodically broadcasted, it is also incremented. The mobile telephone continues to receive the incremented REGID values and compares the current REGID value against the stored REGID value. When the differential between the current and stored REGID values, determined according to a predetermined algorithm, equals or exceeds the REGINCR value being broadcast to the mobile telephone, an Autonomous Registration is initiated by the mobile telephone. The REGID value at the time the Autonomous Registration is initiated is saved as the new stored REGID value. Before Autonomous Registration is completed, the mobile telephone performs a System Access Attempt in accordance with EIA/TIA 553. The first step in the System Access Attempt is a rescan for the strongest available forward control channel. Once the mobile telephone locks onto and seizes the strongest control channel, it completes Autonomous Registration.
In typical macrocellular systems, incoming calls are handled in the following manner. When a caller dials the number of a mobile telephone, the incoming call is routed to the MTSO of the macrocellular network which is the home network of the mobile telephone. The MTSO receives the incoming call and then attempts to locate the desired mobile telephone. The MTSO locates the mobile telephone via a Home Location Register (HLR). The HLR, which may be part of the MTSO or a separate device, maintains a list of the mobile telephones which are subscribers of the macrocellular system. The HLR also maintains a list of the current registration of the mobile telephone. The HLR may be, for example, a microprocessor based device. The registration of the mobile telephone generally will be one of two states: home registered; or IS-41 registered.
Home registered means that the mobile telephone is registered as being located within the macrocellular network. IS-41 registered means that the mobile telephone is registered with another cellular system in accordance with the industry standard EIA/TIA IS-41 Cellular Radiotelecommunications Intersystem Operations specification (EIA/TIA IS-41). EIA/TIA IS-41 provides an industry standard interface for communication between two separate macrocellular systems. For example, every cellular system is assigned a unique system identification number (SID) by the Federal Communications Commission (FCC). The SID is regularly broadcasted by the cells of a given cellular system. If a mobile telephone transitions between two cellular systems having different SIDs, the mobile telephone will detect the change of SID and initiate an Autonomous Registration in the new cellular system if the cellular system is configured for Autonomous Registration. The Autonomous Registration, in turn, will generally cause an IS-41 registration of the mobile telephone, thereby enabling the mobile telephone's home cellular system to continue delivering incoming calls via the IS-41 interface. If Autonomous Registration is not enabled in the cellular system, there will generally be no IS-41 registration even if the mobile telephone detects a change in SIDs. The transition of mobile telephone between two macrocells of the same cellular system will not create a change of SIDs and, thus, there would be no IS-41 registration in this circumstance. IS-41 registered is commonly referred to as "network registered."
If a mobile telephone is home registered, the MTSO will receive a message from the HLR indicating home registration. As a result, the MTSO will send a page signal to the macrocells associated with the MTSO and the macrocells will broadcast the page signal to notify the intended mobile telephone of the incoming call. If the intended mobile telephone receives the page signal, it will send a page response signal in response. The macrocellular system will then set up the incoming call.
If a mobile telephone is IS-41 registered, the MTSO will receive a routing number for the location of the mobile telephone as a result of EIA/TIA IS-41 procedures. This may be the case, for example, when a mobile telephone transitions between cellular systems having different SIDs. In this situation, the MTSO will not broadcast a page signal for the incoming call. The MTSO will, however, deliver the incoming call directly to the mobile telephone via a standard PSTN connection. The routing number is delivered to the HLR from a Visitor Location Register (VLR) associated with the cellular network where the mobile telephone is currently located. The VLR is similar to the HLR except that the VLR has no home customers and only registers mobile telephones that are currently within the cellular network associated with the VLR. The VLR may be, for example, a microprocessor based device and is generally connected to the HLR via an EIA/TIA IS-41 connection.
Outgoing calls from a mobile telephone, whether home registered or IS-41 registered, are handled in accordance with the specifications of EIA/TIA 553 by the cellular system where the mobile telephone is currently located.
In a typical cellular system, as a mobile telephone travels along a path that passes from one cell to another, a handoff occurs if the mobile telephone is active, i.e., the mobile telephone is attempting a telephone call or handling an existing call. The handoff action is controlled by the MTSO which monitors the signal strength received from the mobile telephone. The handoff command is typically generated when the signal received by the mobile telephone falls below a preselected signal strength, thus indicating that the mobile telephone is at the cell boundary of one cell and requires a transfer of the cellular service to an adjacent cell which is able to receive a signal from the mobile above the preselected signal strength.
As a mobile telephone passes from one cell to another cell, the handoff command instructs the new cell which the mobile telephone is entering to begin transmitting at a frequency which is different from the frequency which was transmitted by the cell from which the mobile telephone is exiting. This procedure is followed as the mobile telephone passes into each next successive, adjacent cell. The assigned frequencies of each adjacent cell are different, and such assigned frequencies are not repeated except for cells that are far enough away from each other so that no interference problems will occur.
The commercial success of cellular service has forced cellular carriers to substantially increase the capacity of cellular systems. Accordingly, cellular system providers desire to expand their systems to serve an increasing number of customers within a particular geographic location. Various solutions have been proposed to increase the capacity of cellular systems and thereby address the foregoing need. For example, a typical macrocell may be split into four smaller cells, each with a radius of half the radius of the original macrocell, thereby permitting cellular service to be increased four fold.
Another approach to increasing the capacity of cellular systems is to add additional cell sites to the cellular system to provide additional cellular service in small geographic areas having heavy cellular subscriber usage. These smaller cell sites are typically known as microcell sites. Microcells operate functionally similar to a traditional macrocellular or main cellular system, but only provide cellular service for a small geographic area. The microcell sites are typically directly connected to the macrocellular system using coaxial transmission lines, microwave links or an optical fiber cable network, via a nonstandard protocol which is proprietary to the manufacturer of the macrocellular equipment. Microcells, in general, improve frequency reuse by concentrating frequency channels in the area of greatest subscriber demand and allowing frequency reuse within a particular geographic area.
While cells may be subdivided into smaller cells to provide cellular service for an increasing number of customers, shrinking cell sites create additional considerations and problems. For example, the rate at which mobile telephones move through the cell and the non-uniformity of the electromagnetic field generated in the cell affect the performance of a microcell system. Both factors relate to the time required to determine the relative location of the mobile telephone and to process a handoff of the mobile telephone from the transceiver station of one microcell where the mobile telephone is currently located but is preparing to exit, to the transceiver station of another microcell where the mobile telephone is entering. Naturally, the smaller the cell, the greater the number of handoffs that are required as a mobile telephone moves through the cellular system. If a handoff is required, one or more candidate microcells must be queried for their idle channel status and for a verification of the mobile telephone's signal strength in that candidate microcell. The processing of the decision, status, and verification usually requires the intervention of higher level system control functions in addition to the control function in the serving and candidate microcells. In addition, the mobile telephone must be instructed to tune to a frequency available in the candidate microcell and verification of its presence after the handoff must be made by the candidate cell. Thus, the increased number of handoffs within a microcell system requires a significant amount of time for handoff processing.
In addition, since microcellular systems are usually directly connected to the macrocellular system via a nonstandard proprietary protocol, it is impossible to integrate or include a generic microcell system to off-load cellular service from the macrocellular systems. Generally, the microcellular equipment must be from the manufacturer of the macrocellular equipment for a particular cellular system. The difficulty of competing products to provide cellular service results in microcellular systems being expensive, difficult to integrate with other cellular systems and presents problems with incoming call delivery to a mobile telephone located in the microcellular system.
In a typical macrocellular system which employs a microcellular system to off-load cellular subscribers in an area of heavy subscriber usage, missed incoming calls are a problem. In particular, if a cellular system provider integrates a current microcellular system into an existing macrocellular system and uses microcellular equipment that was not manufactured by the macrocellular equipment manufacturer, then incoming calls may be missed. The missed calls result when a mobile telephone transitions from the macrocellular system to the microcellular system. When the mobile telephone enters the microcellular system, it automatically locks onto a frequency of the microcell in accordance with EIA/TIA 553 because the microcellular control channel is the strongest signal available to the mobile telephone at that time. As a microcellular system is designed to off-load traffic from the macrocellular system, the microcellular system must broadcast the same SID as the macrocellular system. Thus, because the mobile telephone detects no change in SIDs, there is no registration of the mobile telephone with the macrocellular system as being in the microcellular system upon the transition into the microcellular system. Accordingly, the location of the mobile telephone will be unknown to the macrocellular system while the mobile telephone remains in the microcellular system.
As a result, on an incoming telephone call, the macrocellular system HLR will still have the mobile telephone listed as being home registered and a page signal will be broadcast on macrocellular frequencies while the mobile telephone is locked onto a microcellular frequency. Consequently, the mobile telephone located in the microcellular network and locked onto a microcellular frequency will miss the page signal and the incoming telephone call.
In current microcellular systems having equipment manufactured by the manufacturer of the macrocellular equipment and being directly connected to the macrocellular system, such as by coaxial transmission lines, microwave links or an optical fiber cable network, via a matched nonstandard proprietary interface, incoming calls may not be missed. In this circumstance, the new microcells are essentially additional macrocells. A mobile telephone located within this type of microcellular network will be tracked by the HLR as being home registered and the MTSO will broadcast a page signal for incoming calls. The page signal will be sent to both the macrocells and the microcells because the microcells are directly connected to the macrocellular system. If the intended mobile telephone provides a page response signal, the incoming call will be set up. In this situation, however, the cellular system provider is constrained to purchase microcellular equipment from the manufacturer of the macrocellular equipment due to the nonstandard proprietary interface between the equipments, thus making the microcellular equipment expensive.
Thus, current microcellular systems which are integrated into existing macrocellular systems must have the same manufacturer as the existing macrocellular equipment to implement the nonstandard proprietary interface between the macrocellular and microcellular equipment and ensure incoming call delivery. Such microcellular systems, however, are unduly expensive. If microcellular equipment not manufactured by the maker of the macrocellular equipment is used, then incoming calls may be missed if a mobile telephone is located in the microcellular system.
Thus, it is desirable to provide for easy and inexpensive off-load cellular service for a macrocellular system by establishing an off-load microcellular system in a small area which requires a high concentration of cellular service. For example, a large office building or shopping mall may require a significant amount of cellular service which could be easily provided using a microcellular system since mobile telephone users typically establish cellular service while walking from office to office in a building or from store to store in a shopping mall. Thus, mobile telephones in these small areas are less likely to rapidly move across large geographic areas, making use of a microcellular system ideal.
In addition, there is currently no method of off-loading cellular service from a macrocellular system to another macrocellular system or to an off-load microcellular system without reconfiguring the macrocellular system to ignore the cellular service in the microcellular system which is providing cellular service in the same area where the macrocellular system had previously provided the cellular service. It is, therefore, desirable to provide this additional or off-load microcellular service for customers in a high concentration area of a macrocellular system without reconfiguring the main cellular system, thereby providing off-load microcellular service which is transparent to the main cellular system.
It is also desirable to provide off-load cellular service for customers in a high concentration area served by a main cellular system by replacing the macrocellular system in the high concentration area with off-load microcellular service without reconfiguring the macrocellular system, thereby providing off-load microcellular service which is transparent to the macrocellular system.
Further, it is desirable to off-load cellular service from a main cellular system using, for example, an off-load microcellular system to provide cellular service in an area of high concentration, without requiring all traffic in the microcellular system to interface with the MTSO. The reason that it is undesirable for the off-load microcellular system to always be required to interface with the MTSO is to reduce the processing burden of the MTSO and because no standard interface has been developed which allows the off-load microcellular system to be easily integrated with the MTSO without the off-load microcellular system being required to implement the specific interface requirements of the MTSO.
It is also desirable to support industry standard methods for mobile telephone registration and incoming call processing between the macrocellular system and the off-load microcellular system. Since various vendors provide MTSOs for cellular service using different interface protocols, an off-load microcellular system that did not support industry standards would be required to know the interface protocol used by each cellular system. Such an off-load microcellular system would be complex and not easily integratable into an already existing cellular system.
Finally, it is desirable that a mobile telephone which is being serviced by the off-load microcellular system be able to establish telephone service with telephone equipment which is located outside the microcellular system. Thus, even though the off-load microcellular system is transparent to the main cellular system, the off-load microcellular system must still be able to provide telephone service with telephone equipment located outside the serving area of the off-load cellular system, such as a PSTN.